1. Field of the Invention
This invention relates to external-coupled-cavity diode lasers. Accordingly, it is a general object of this invention to provide new and improved diode lasers of such character.
2. General Background
The following publications supply a background for the invention. Their specific pertinence will become more apparent from a reading of this specification.
1. M. Kitamura, M. Seki, M. Yamaguchi, I. Mito, S. Murata, and K. Kobayashi, "Single-Longitudinal-Mode 1.3 .mu.m DFB-DC-PBH Laser Diodes", presented at the Topical Meeting on Optical Fiber Communication (New Orleans, La.), paper No. MF2, Jan. 23-25, 1984, hereinafter termed "Kitamura et al.".
2. W. T. Tsang, N. A. Olsson, and R. A. Logon, "High Speed Single-Frequency Modulation with Large Tuning Rate and Frequency Excursion in Cleaved-Coupled-Cavity Semiconductor Lasers", Appl. Phys. Lett., vol. 42, p. 650 (1983), hereinafter termed "Tsang et al".
3. L. A. Coldren, C. A. Burrus, K. J. Ebeling, T. L. Koch, R. G. Swartz, and J. E. Bowers, "Verification of Coupling Gap Dependence in Coupled Cavity Lasers", presented at the Topical Meeting on Optical Fiber Communication (New Orleans, La.), paper No. MF5, Jan. 23-25, 1984, hereinafter termed "Coldren et al. #1".
4. L. A. Coldren, R. G. Swartz, K. J. Ebeling, and C. A. Burrus, "Stable Single-Wavelength Operation of Two-Section Coupled-Cavity Lasers Under Modulation Using Feedback Control", presented at the Topical Meeting on Optical Fiber Communication (New Orleans, La.), paper No. MF6, Jan. 23-25, 1984, hereinafter termed "Coldren et al. #2".
5. C. Lin, C. A. Burrus, and L. A. Coldren, "Characteristics of Single-Longitudinal-Mode Selection in Short-Coupled-Cavity Injection Lasers", IEEE J. Lightwave Tech., vol. LT-2, p. 544 (1984), hereinafter termed "Lin et al.".
6. K.-Y. Liou, S. W. Grandlund, C. B. Swan, C. A. Burrus, R. A. Linke, I. P. Kaminow, and P. Besomi, "Single-Longitudinal-Mode Operation of GRIN Rod External-Coupled-Cavity Semiconductor Lasers", presented at the Topical Meeting on Optical Fiber Communication (New Orleans, La.), paper No. TUL2 Jan. 23-25, 1984, hereinafter termed "Liou et al.".
7. D. M. Fye, "An Optimization Procedure for the Selection of Diode Laser Facet Coatings", IEEE J. Quantum Electron., vol. QE-17, p. 1950 (1981), hereinafter termed "applicant's earlier paper".
In order to optimize the use of a fiber optic communication system, the optical source should emit at the wavelength for which attenuation in the optical fiber is minimized. In addition, the spectral width of the modulated source should be sufficiently narrow to avoid significant pulse distortion due to fiber dispersion. Fiber dispersion, as is known, is a characteristic due to different spectral components of a pulse that travel along a fiber at different speeds. Typical optical fibers exhibit minimum attenuation at a wavelength of 1.55 .mu.m, but fiber dispersion at that wavelength is sufficient to limit the performance of high speed transmission systems unless diode lasers with single longitudinal mode emission are used as optical sources. The development of mode-stabilized diode lasers is therefore desired for high speed, long distance optical communication applications.
Several different types of mode-stabilized diode lasers exist. They include distributed feedback (DFB) lasers, cleaved-coupled-cavity (C.sup.3) lasers, and external-coupled-cavity lasers. All of these devices achieve single-mode operation through the introduction of wavelength-dependent cavity losses which allow only a single longitudinal mode to reach lasing threshold. While DFB lasers, according to Kitamura et al., supra, provide excellent suppression of spurious longitudinal modes, their fabrication requires fine-line lithography which reduces the yield and increases the production cost. C.sup.3 lasers, according to Tsang et al., supra, are three-terminal devices, the operation of which are greatly complicated by the need to control two modulation currents in order to obtain high-speed, single-mode emission over realistic changes of device characteristics with aging and temperature fluctuations, as suggested by Coldren et al. #1, supra. In addition, the operating characteristics of C.sup.3 lasers are critically dependent on the length of the gap between two laser sections, as taught by Coldren et al. #2, supra. The gap length is extremely difficult to control during device fabrication, resulting in unpredictable device performance, reduced yields and increased cost.
External-coupled-cavity lasers use a passive external cavity in combination with a simple Fabry-Perot laser to obtain the wavelength-selective cavity loss necessary for single-mode operation. External cavities consisting of planar reflectors, as suggested by Lin et al., supra, or mirror-coated graded index rods, as suggested by Liou et al., supra, have been reported to yield single-mode emission over reasonable temperature ranges.
Significantly, a disadvantage of previously reported external-coupled-cavity laser structures is that the critical alignment of the external reflectors is difficult to achieve in an efficient production process. In addition, previous structures failed to realize the significant improvement in spurious mode rejection and reduced operating current that can be obtained by using high reflectivity facet coatings on the diode laser as taught herein.